Equal energy selection in an electron microscope using electron optics



June 14, 1966 HIROSHI WATANABE ET AL EQUAL ENERGY SELECTION IN AN ELECTRON MICROSCOPE USING ELECTRON OPTICS Filed March 25, 1965 F/rsf def/ecf/ng m/fage Second def/ecf/hg voltage Q '5 Hircsl'ai (Disnab Ru ozj U yed a.

ATToRnEy under examination.

United States Patent Claims. Cl. 250-495 The present invention relates to energy-selecting electron microscopes.

Electron rays forming an electron image in electron microscopes generally include electrons of different energy values. Recent progress of the accelerating-voltage stabilizer has made it possible to employ an incident beam including electrons of a substantially definite energy value. However, even where an incident electron beam is obtained which includes electrons of a definite energy value, the beam undergoes not only the elastic but also the inelastic scattering collisions of the specimen Accordingly the beam after it has passed through the specimen includes electrons of dif ferent energy values, which are incoherent to each other. On the other hand, the focal length of electron lenses depends upon the energy of electrons passing therethrough. Accordingly, inelastically scattered electrons sporadically reach the image plane at points around the image formed therein by elastically scattered electrons, and thus impairs the quality of the image.

In view of the above difliculties, the present invention is intended to improve the resolving power of an electron microscope through exclusion of inelastically scattered electrons to form an image solely by elastically scattered electrons. The invention is also intended to provide a novel and unique means for excluding elastically scatteredelectrons, if desired, to form an electron image solely of inelastically scattered electrons, thereby enabling experimental investigations to be conducted, for instance, on the inelastic electron-scattering process and the behavior of inelastically scattered electrons, or on the influence of inelastically scattered electrons upon the electron image.

According to the present invention, there is provided an energy-selecting electron microscope which comprises components arranged between the objective lens and the imaging plane of the microscope including first deflector means for scanning the electron image of a specimen under examination formed by the objective lens, a first aperture diaphragm or a fine slit arranged in the image plane of the objective lens, means for energyanalyzing electron rays forming respective sections of said image of the specimen and passing through said first aperture diaphragm in time sequence, a second aperture diaphragm or a fine slit adapted to pass only those electron rays having a particular energy value as selected from the electron rays energy-analyzed by said analyzing means, and second deflector means for defleeting said electron rays of the same energy passing through said second aperture diaphragm or fine slit in a manner so that said electron rays are directed back to the points corresponding to the respective image sections to form a final image of the specimen in a proper axial position.

The present invention will now be described in detail with reference to the accompanying drawing, which schematically illustrates an instrument embodying the present invention.

Referring to the drawing, the electron microscope includes an objective lens 4, a first scanning deflector 5, a first aperture diaphragm 6, including a small aperture 7 of the specimen as a whole.

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or a slit, an analyzing lens 8 for energy 'analyzsis of electron rays, a second aperture diaphragm 9 including a small aperture or a slit, a cylindrical electron lens 12, a second scanning deflector 13, and an imaging plane 14. Reference numeral 1 designates an electron beam emitted from an electron gun (not shown) to pass through a specimen 2 to be examined, which is placed in front of the objective lens 4. The electron beam 1 is initially monoenergetic having a certain energy E but the beam is scattered by the specimen during passage therethrough. The scattered beam 3 including electrons of different energy values E E E proceeds to form an image 7 in the plane of the first aperture diaphragm 6. Numerals 10 and 11 designate images formed by energy-analyzed electron rays and numeral 15 designates a fiinal image formed in the imaging plans 14.

In use, a first deflecting voltage V as shown in the left-hand portion of the drawing is applied to the first deflector 5 to scan the image 7 of the specimen formed in the plane of the first aperture diaphragm 6. It is assumed that the image 7 positioned for example at 7' at the initial instant of 23:0 will assume another position 7" at the instant of i=7. The electron ray passing through the slit in the first aperture diaphragm 6 includes electrons forming one end of-the image (or the tail end of the arrow) at the initial instant of t=0 and the other end of the image (or the pointed end of the arrow) at the instant of 1:7. The electrons passing through the slit in the first aperture diaphragm 6 to form the respective portions of the image have different energy values E E E as indicated above, and are subjected to energy analysis in the analyzing lens 8. The analyzing lens 8 as illustrated takes the form of an electrostatic unipotentioal lens having an elongated rectangular opening and having an extraordinarily large chromatic aberration constant suitable for energy analysis of electron rays. It is to be understood, however, that the analyzing lens is in essence a form of electron prism effective to disperse an electron ray in accordance with the energy difference between the electrons. It will be apparent that a pair of electron deflector plates may also serve the purpose with success. analyzing lens is that despite its high resolving power it can hardly cause such a'high degree of electron deflection as to enable the observer to discern the image 7 This is partly because of the limited divergence in energy of the scattered electron beam. To meet this situation, the first deflector 5 is employed to break up the image 7 of the specimen into minute sections, and electrons forming each of such minute image sections are subjected to energy analysis. In this manner, an electron image formed by electrons of the same energy can be obtained by energy analysis despite of any substantial size of the image 7. In other words, any of monoenergetic electron rays having respective energy values E E can be selected for observation as long as it is arranged so that the electron rays forming the respective image sections are individually energy-analyzed.

In the embodiment illustrated, it is noted that, of all the electron rays which form the tail end of the image positioned at 7' at the instant of t=0, only the electron ray including elastically scattered electrons and hence of of the energy E, is projected through the slit formed in the second aperture diaphragm 9, as indicated at 10', whereas the electron ray including inelastically scattered electrons of the energy E is projected to the plane of the second aperture diaphragm 9 at a point off the slit formed therein, as indicated at 11'. Similarly, of all the electron rays forming the pointed end of the image positioned at 7" at the instant of i=r-ih6 ray of the energy level E Patented June 14, 1966' One problem with the' is projected to the second diaphragm 9 at point 10" whereas the ray of the energy E is projected to point 11". Thus, the electron rays passing through the slit in the second diaphragm include elastically scattered electrons only, having the energy E It will now be appreciated that, with this arrangement, only those electron rays including elastically scattered electrons of the energy E, can be effectively selected for observation from all the electron rays forming the respective image sections, despite any substantial size of the image 7 of the specimen.

The electron rays of the energy E, after passage through the second aperture diaphragm 9 are scanned by the second deflector 13, which is supplied with a second deflecting voltage V as illustrated in the left-hand portion of the drawing. The deflecting voltage V is in inverse time sequence to the first deflecting voltage V It will readily be understood that a final image 15 of the specimen under examination is formed in the imaging plane 14 by the electron rays subjected to the scanning action of the second deflector 13, and that the final image thus obtained is of extraordinarily high resolution since it is formed of elastically scattered electrons of the same energy E The cylindrical electron lens 12, arranged beneath the second aperture diaphragm, is provided to serve the purpose of improving the quality of the final image by correcting the astigmatism of the analyzing lens 8. When it is desired to obtain a still higher magnification, a suitable magnifying lens system, not shown, may be combined to magnify the final image 15.

It will readily be appreciated that with the electron microscope of the present invention any electron rays of an energy level other than E may be selected with ease by adjusting the resolving power of the analyzing lens 8 or the position of the slit in the second aperture diaphragm 9 to obtain an electron image formed by the monoenergetic electron rays. Accordingly, not only is it possible to obtain an electron image of high resolution by employing elastically scattered electrons only, but also an electron image may be formed by inelastically scattered electrons only, thus providing an effective means of investigating the effects of inelastically scattered electrons upon an electron image as obtained with conventional electron microscopes. The eflects of inelastically scat tered electrons on an electron image can now be experimentally ascertained though previously they have been dealt with merely upon the basis of theoretical presumptions.

As apparent from the foregoing, the electron microscope of the invention has a number of merits and a wide range of applications as compared with previous electron microscopes. Namely, it has an improved resolving power and enables close and accurate observations since it forms an electron image of high resolution by use of monoenergetic electro nrays. In addition, it provides an effective means of experimentally elucidating the influence upon the image resolution of the different scattering processes occurring in the specimen due to'collision of electrons therein. It will be appreciated that the electron microscope of the invention is extremely valuable for researches in microscopial fields.

What is claimed is:

1. An energy-selecting electron microscope having electron beam generating means, a specimen and an objective lens comprising components arranged between said objective lens and the imaging plane of the microscope including first deflector means for scanning an electron image of the specimen under examination formed by the objective lens, a first aperture diaphragm arranged in the image plane of the objective lens and formed with a narrow aperture, means for energy-analyzing electron rays forming respective sections of said image of the specimen and passing through said first aperture diaphragm in time sequence, a second aperture diaphragm adapted to pass only those electron rays having a particular energy value as selected from the electron rays energy-analyzed by said analyzing means, and second deflector means for deflecting said electron rays of the same energy passing through said second aperture diaphragm with an inverse time sequence to said first deflector means so that said electron rays are directed back to the points corresponding to the respective image sections to form a final image of the specimen in a proper axial position.

2. An energy-selecting electron microscope comprising components arranged between the objective lens and the imaging plane of the microscope and including first deflector means for scanning the electron image of a specimen under examination formed by the objective lens, a first aperture diaphragm arranged in the image plane of the objective lens and formed with a narrow aperture, means for energy-analyzing electron rays forming respective image sections of said image of the specimen and passing through said first aperture diaphragm in time sequence corresponding to the deflection cycle of said first deflector means, a second aperture diaphragm adapted to pass only those electron rays having a particular energy level as selected from the electron rays energy-analyzed by said analyzing means, means for correcting the astigmatism of said analyzing means for the electron rays passing through said second aperture diaphragm, and second deflector means for deflecting the corrected electron rays with an inverse time sequence to said first deflector means so as to direct them back to the points corresponding to .the respective image sections to form a final image of the specimen in a proper axial position.

3. An energy-selecting electron microscope having electron beam generating means, a specimen and an objective lens comprising components arranged between said objective lens and the imaging plane of the microscope including first deflector means for scanning the electron beam having the electron image of the specimen under examination formed by the objective lens, first selecting means arranged in the path of said electron beam for passing sections of said scanned electron beam in time sequence corresponding to the deflection cycle of said first deflector means,

means for separating said sections passed by said first selecting means into monoenergetic electron rays,

second selecting means for selecting one of said monoeneirgetic electron rays to the exclusion of the others, an

second deflector means for deflecting the sections of said one monoenergetic electron ray with an inverse time sequence to said first deflector means so that said sections are directed back to the points corresponding to the original image thereby forming a final image of the specimen.

4. An energy-selecting electron microscope having electron beam generating means, a specimen and an objective lens comprising components arranged between said objective lens and the imaging plane of the microscope including first deflector means for scanning the electron beam having the electron image of the specimen under examination formed by the objective lens,

first selecting means arranged in the path of said electron beam for passing sections of said scanned electron beam in time sequence corresponding to the deflection cycle of said first deflector means, means for separating said sections passed by said first selecting means into monoenergetic electron rays,

second selecting means for selecting one of said monoenelrgetic electron rays to the exclusion of the others, an

second deflector means for deflecting the sections of said one monoenergetic electron ray with in inverse time sequence to said first deflector means so that said sections are directed back to the points corresponding to the original image thereby forming a final image of the specimen,

said second selecting means being in the form of adjustable aperture diaphragms capable of transverse adjustment with respect to the longitudinal axis of the microscope.

5. An energy-selecting electron microscope having electron beam generating means, a specimen and an objective lens comprising components arranged between said objective lens and the imaging plane of the microscope including first deflector means for scanning the electron beam having the electron image of the specimen under examination formed by the objective lens,

first selecting means arranged in the path of said electron beam for passing sections of said scanned electron beam in time sequence corresponding to the deflection cycle of said first deflector means, means for separating said sections passed by said first selecting means into monoenergetic electron rays,

second selecting means for selecting one of said monoenergetic electron rays to the exclusion of the others, and

second deflector means for deflecting the sections of said one monoenergetic electron ray with an inverse time sequence to said first deflectormeans so that said sections are directed back to the points corresponding to the original image thereby forming a final image of the specimen,

said second selecting means being in the form of adjustable aperture diaphragms capable of transverse adjustment with respect to the longitudinal axis of the microscope,

said first and second deflector means being in the form of oppositely disposed electrostatic deflection plates.

6. An energy-selecting electron microscope having electron beam generating means, a specimen and an objective lens comprising components arranged between said objective lens and the imaging plane of the microscope including first deflector means for scanning the electron beam having the electron image of the specimen under examination formed by the objective lens,

first selecting means arranged in the path of said electron beam for passing sections of said scanned electron beam in time sequence corresponding to the deflection cycle of said first deflector means, means for separating said sections passed by said first selecting means into monoenergetic electron rays,

second selecting means for selecting one of said monoenergetic electron rays to the exclusion of the others, and second deflector means for deflecting the sections of said one monoenergetic electron ray with an inverse time sequence to said first deflector means so that said sections are directed back to the points corresponding to the original image thereby forming a final image of the specimen, said second selecting means being in the form of adjustable aperture diaphragms capable of transverse adjustment with respect to the longitudinal axis of the microscope,

said first and second deflector means being in the form of oppositely disposed electrostatic deflection plates,

said means for separating said sections passed by said first selecting means into monoenergetic electron rays being in the form of an electrostatic unipotential lens having a large chromatic aberration constant.

7. An energy-selecting electron microscope having electron beam generating means, a specimen and an objective lens comprising components arranged between said objec tive lens and the imaging plane of the microscope including first deflector means for scanning the electron beam having the electron image of the specimen under examination formed by the objective lens,

first selecting means arranged in the path of said electron beam for passing sections of said scanned electron beam in time sequence corresponding to the deflection cycle of said first deflector means, 5 means for separating said sections passed by said first selecting means into monoenergetic electron rays, sceond selecting means for selecting one of said monoenergetic electron rays to the exclusion of the others, lens means for correcting the astigmatism of said 10 separating means for the electron rays passing through said second selecting means, and

second deflector means for deflecting the sections of said one monoenergetic electron ray with an inverse time sequence to said first deflector means so that said sections are directed back to the points corresponding to the original image thereby forming a final image of the specimen,

said second selecting means being in the form of adjustable aperture diaphragms capable of transverse adjustment with respect to the longitudinal axis of the microscope.

-8. An energy-selecting electron microscope having electron beam generat-ing means, a specimen and an objective lens comprising components arranged between said objective lens and the imaging plane of the microscope ineluding first deflector means for scanning the electron beam having the electron image of the specimen under examination formed by the objective lens, first selecting means arranged in the path of said electron beam for passing sections of said scanned electron beam in time sequence corresponding to the deflection cycle of said first deflector means, means for separating said sections passed by said first selecting means into monoenergetic electron rays, second selecting means for selecting one of said monoenergetic electron rays to the exclusion of the others, lens means for correcting the astigmatism of said separating means for the electron rays passing through said second selecting means, and second deflector means for deflecting the sections of said one monoenergeti'c electron ray with an inverse time sequence to said first deflector means so that said sections are directed back to the points corresponding to the original image thereby forming a final image of the specimen, said second selecting means being in the form of adjustable aperture diaphragms capable of transverse adjustment with respect to the longitudinal axis of the microscope,

said means for separating said sections passed by said first selecting means into monoenerge-tic electron rays being in the form of an electrostatic unipotential lens having a large chromatic aberration constant.

9. An energy-selecting electron microscope having electron beam generating means, a specimen and an objective lens comprising components arranged between said objective lens and the imaging plane of the microscope including first deflector means for scanning the electron beam having the electron image of the specimen under examination formed by the objective lens,

first selecting means arranged in the path of said electron beam for passing sections of said scanned electron beam in time sequence corresponding to the deflection cycle of said first deflector means,

means for separating said sections passed by said first 7O selecting means into monoenergetic electron rays,

second selecting means for selecting one of said monoenergetic electron rays to the exclusion of the others, lens means for correcting the astigmatism of said separating means for the electron rays passing through said second selecting means, and

7 8 second deflector means for deflecting the sections of sponding to the original image thereby forming a said one monoenerge-tic electron ray with an inverse final image of the specimen, time sequence to said first deflector means so that said second selecting means being in the form of adsaid sections are directed back to the points correjustable aperture diaphragms capable of transverse spending to the original image thereby forming a adjustment with respect to the longitudinal axis of final image of the specimen. the microscope,

'10. An energy-selecting electron microscope having said means for separating said sections passed by said electron beam generating means, a specimen and an obfirst selecting means into monoenergetic electron rays jective lens comprising components arranged between said being in the form of an electrostatic unipotential lens objective lens and the imaging plane of the microscope 10 having a large chromatic aberration constant. including first deflector means for scanning the electron beam References Cited y the Examine! having the electron image of the specimen under UNITED STATES PATENTS examination formed by the objective lens, K first selecting means arranged in the path of said elec- 539L243 1/1946 Hinder 250495 tron beam for passin sections of said scanned elec- 3/1946 Von Bones 250495 2 429 558 10/1947 Marton 250-49.5 X tron beam 1n time sequence eorrespondin to the de- 2,444,710 7/ 1948 Ramberg 250-495 $313235?ZZilLl t Said first 2,494,442 1/195" Le Pools 250-495 selecting means iITtO monoenergetic electron rays 289416O 7/1959 Sheldon 250-495 3,134,899 5/1964 Guyenot et a1 25049.5 second selectlng means for selecting one of said monoenergetic electron rays to the exclusion of the others, FOREIGN PATENTS and 760,135 11/1953 Germany.

second deflector means for deflecting the sections of said one monoenergetic electron ray with an inverse RALPH Prlmary Exammertime sequence to said first deflector means so that G. E. MATTHEWS, H, S, MILLER, said sections are directed back to the points corre- Assistant Examiners. 

1. AN ENERGY-ELECTRON ELECTROM MICROSCOPE HAVING ELECTRON BEAM GENERATING MEANS, A SPECIMEN AND AN OBJECTIVE LENS COMPRISING COMPONENTS ARRANGED BETWEEN SAID OBJECTIVE LENS AND THE IMAGING PLANE OF THE MICROSCOPE INCLUDING FIRST DEFLECTOR MEANS FOR SCANNING AN ELECTRON IMAGE OF THE SPECIMEN UNDER EXAMINATION FORMED BY THE OBJECTIVE LENS A FIRST APERTURE DIAPHRAGM ARRANGED IN THE IMAGE PLANE OF THE OBJECTIVE LENS AND FORMED WITH A NARROW APERTURE, MEANS FOR ENERGY-ANALYZING ELECTRON RAYS FORMING RESPECTIVE SECTIONS OF SAID IMAGE OF THE SPECIMEN AND PASSING THROUGH SAID FIRST APERTURE DIAPHRAGM IN TIME SEQUENCE, A SECOND APERTURE DRAPHRAGM ADAPTED TO PASS ONLY THOSE ELECTROM RAYS HAVING A PARTICULAR ENERGY VALUE AS SELECTED FROM THE ELECTRON RAYS ENERGY-ANALYZED BY SAID ANALYZING MEANS, AND SECOND DEFLECTOR MEANS FOR DEFLECTING SAID ELECTRON RAYS OF THE SAME ENERGY PASSING THROUGH SAID SECOND APERTURE DIAPHRAGM WITH AN INVERSE TIME SEQUENCE TO SAID FIRST DEFLECTOR MEANS SO THAT SAID ELECTRON RAYS ARE DIRECTED BACK TO THE POINTS CORRESPONDING TO THE RESPECTIVE IMAGE SECTIONS TO FORM A FINAL IMAGE OF THE SPECIMEN IN A PROPER AXIAL POSITION. 